Various processes and systems have been used for producing ethanol from feedstock. For example, in some prior systems, ethanol is produced by fermentation, yielding a stillage (beer) with an ethanol concentration of up to 18%, which is subsequently concentrated in three steps: (1) distillation in a beer column, increasing the ethanol concentration up to 65%, followed by (2) processing in a stripper/rectifier column further increasing the ethanol concentration to around 90 vol %, and (3) a molecular-sieve-based dehydration (also referred to as pressure swing adsorption) to a target ethanol concentration of 99.0 to 99.95 vol %.
In the stripper/rectifier column, a mixture of high boiling components including propanol, butanol, and isomeric pentanols (also referred to as fusel oil) may need to be removed in a side draw to avoid accumulation therein. In some prior systems, pressurized product vapor coming from the molecular sieve unit (MSU) is acidic mainly due to carbonic acid, and the pressurized product vapor may need to be de-acidified in an acid removal column (ARC). As pressure decreases and temperature increases, carbonic acid dissociates into CO2 (a non-condensable gas) and water. The ARC is a counter-current vapor/liquid adsorption/desorption unit with vapor entering the bottom of the column flowing upward and liquid entering the top of the column flowing downward. Through the counter-current flow, the vapor condenses and heats up the liquid, thus driving out the CO2 gas.